The nuclear regulatory agencies of the United States have established numerous testing and diagnostic requirements to ensure that all safety related, position changeable motor operated valves ("MOV's") are capable of operating during design basis events. One such recent requirement calls for an initial test program for establishing and implementing correct control switch settings on all safety related MOV'S within a nuclear plant or facility. A correct MOV switch setting allows adequate torque/thrust to close the valve against a "differential pressure force". Differential Pressure may be defined as the fluid system, internal pressure which the motor operator must overcome when the valve is opening or closing and is near the full closed position, in which position the system pressure will usually be the highest on the pump side of the valve and minimal on the downstream side. "Differential pressure force" may be defined as the product of the differential pressure multiplied by valve area then by a representative friction coefficient, plus the product of the line pressure multiplied by the cross sectional area of the stem. When setting the switch, a "margin" is provided which is, actually, a conservatism in the setting. For example, if the correct switch setting required 15,000 pounds of thrust and the actual setting was 17,000 pounds, the margin would be 2000 pounds.
Following this initial (or baseline) testing, periodic testing is required to identify degradations and to insure that settings remain adequate during the life of the facility.
It should be noted that the MOV actuator is typically controlled by a torque switch and, thus, the adequacy of the torque switch setting depends on the actuator's conversion of torque to thrust. For example, the torque switch is set at "x" foot-pounds in expectation of delivering "y" pounds of thrust to the stem. There is a known concern that torque within the MOV may, due to certain degredations, be developed to a point high enough to trip the torque switch (i.e. "x" foot-pounds, thus shutting off the MOV motor) before the valve has indeed closed or opened (for example, before the stem load reached "y" pounds of thrust). In such a situation, it is typically explained that the allowance or "margin" provided during the baseline test has been "reduced" or extinguished. Previously in the industry, it has, thus, been determined to be a beneficial part of MOV diagnostics to periodically monitor the thrust margin to determine if sufficient thrust remains in the MOV to deliver the thrust required to open or close the valve.
One method for periodically monitoring the thrust margin involves going directly to the valve and measuring the available stem thrust. This approach identifies increases in stem running loads; however, it does not appear to sufficiently identify developing motor or actuator problems. Degradations in the motor or actuator could eventually result in insufficient torque to trip the torque switch. This condition normally leads to tripping thermal overloads or motor burn up.
The present inventor has earlier developed and (his assignee) marketed valve operator diagnostic systems for monitoring of power parameters such as power factor or motor load to detect depletion of thrust margin. Examples of such systems are seen in U.S. Pat. Nos. 4,831,873 and 4,869,102. These systems utilized simulated or calculated stem running loads (thrust) to develop a direct relationship between a power parameter and the stem load during running condition; then used this relationship to set up a threshold value for the power parameter based on a maximum allowable stem thrust depletion; and then periodically monitored the power parameter for comparison to the threshold. Such systems, in effect, substituted the power parameter measurement for the direct measurement (mentioned above) of the stem thrust. Thus, such systems are subject to similar short comings of the first method mentioned above. In an ongoing attempt to address the problems of such valve monitoring, the present inventor has recognized a need for an improved method of monitoring margin in situations where extended intervals exist between tests; it having been recognized by the inventor that the relationships between the power parameter and the stem thrust established by his earlier systems are not maintained after such intervals.